The present invention relates to a method and apparatus for producing a variable threshold level. More particularly, it relates to a variable level threshold generator for establishing the decision threshold level of a composite envelope signal detector in a frequency shift keyed (FSK) demodulator.
In frequency shift keyed signal communication, a binary signal is transmitted by selectively modulating a carrier signal at one of two designated frequencies. The two resulting tone signals produced by modulation with the two frequencies are commonly denoted as "mark" and "space" signals. In a receiver responsive to FSK signals, a demodulator is utilized to determine whether a mark or space signal is being received. The demodulator includes circuitry for converting the received signal into a signal having a predetermined voltage level which is dependent upon the frequency and amplitude of the received signal. This signal is fed to a decision circuit which determines which one of the two anticipated tone signals has been received. Conventionally, the voltage signals for the mark and space tone signals are combined to form a composite envelope signal in which the mark tone is designated by a signal level of a first polarity, and a space tone is designated by a signal level of the opposite polarity. The decision circuit detects the polarity of the incoming signal relative to a threshold value, and produces an output signal indicative of the detected polarity.
Under ideal conditions, the absolute values of the amplitudes of the mark and space tone signals are equal. Therefore, the threshold value utilized by the decision circuit is set to be zero volts, since it is midway between the negative and positive peak amplitude excursions of the composite envelope signal. Due to signal attenuation, however, the amplitude of the two tone signals will not always be the same. This phenomenon, known as selective fading, occurs when one of the mark and tone signals is attenuated more than the other during signal transmission. When this selective fading occurs, it is desirable to maintain the threshold level midway between the peak positive and negative amplitude values of the composite signal, i.e., at the average value of the mark and space tone signals. However, when no data signal is being received, or when one tone signal is transmitted for a relatively long period of time, it is desirable to set the threshold level to be at the statistically most probable value, i.e., zero volts.
One proposed solution for providing a threshold value in an FSK receiver is set forth in U.S. Pat. No. 2,999,925, issued to Thomas. In the threshold computer of the Thomas patent, the negative and positive portions of the composite envelope signal are separated and the two resulting wave forms are capacitively coupled to D.C. restorers of opposite polarities. The D.C. restored wave forms are peak detected utilizing RC networks having predetermined decay rates. The output signals of the two detectors are resistor summed to form the required threshold signal.
One problem associated with threshold computers of the type which utilize RC circuits, such as that disclosed in the Thomas patent, is the dependence of the threshold signal upon the data content, i.e., mark to space signal ratio of the received FSK signal. For example, if a plurality of mark tone signals are consecutively received, i.e., uninterrupted by space tone signals, the RC circuits will begin to discharge the energy stored therein and the threshold signal will decay towards zero volts. If the decay rate of the RC circuits is decreased so that the threshold signal does not decay towards zero as rapidly, the responsiveness of the threshold computer to changes in the peak amplitudes of the composite envelope signal will be correspondingly decreased. Consequently, a compromise must be reached between the decay rate of the threshold signal and the response rate of the computer, since these two rates cannot be independently adjusted.